Unity + Steamworks.NET: GetLaunchCommandLine() does not include “+connect_lobby” when launching through an invite

The title describes the problem. I am trying to launch the game through an invite from a friend. When I accept the invite, it launches the game just fine. But when I try to then read the Command line through SteamApps.GetLaunchCommandLine() it does not contain the "+connect_lobby" that the documentation describes. There is very little I can find on this online.

What situations could cause the game to not start up with that line when it should? Is there an alternative?

I read the command line as intended, like this:

string steamCommandLine = ""; Steamworks.SteamApps.GetLaunchCommandLine(out steamCommandLine, 260); 

Note that the OnGameRichPresenceJoinRequested callback works perfectly as intended.

Edit: Almost forgot to add that we have a custom parameter that does go through. Would it be possible that this parameter overwrites the rest of the command line?

Edit 2: I did a bit more research, and in the steam console (with spew level 4 4) I found that steam executes a url: ExecuteSteamURL: "steam://rungame/<appID>/<steamID64>/GuideHouse". That steamID is exactly what I want, but I am not able to retrieve it in code…